A smart antenna is an array of antenna elements connected to a digital signal processor. Such a configuration dramatically enhances the capacity of a wireless link through a combination of diversity gain, array gain and interference suppression. Increased capacity translates to higher data rates for a given number of users, or more users for a given data rate per user. A smart antenna can also separate signals from multiple users who are separated in space but who use the same radio channel (i.e., center frequency, time-slot, and/or code). This application is known as space-division multiple access (SDMA).
There are still many challenges in the practical implementation of a smart antenna. The digital signal processing is usually performed on the IF or baseband signals. It requires that the signal amplitude and phase information be conveyed properly from the antenna elements to the signal processing stage. In a typical receiver connected to a smart antenna, an independent RF channel is needed for each antenna element. For an N-element antenna array, the total number of RF channels is N.
The cost in terms of hardware and power consumption of such a system is approximately N times those in a single antenna system requiring only a single RF channel. Moreover, antenna arrays with multiple feed lines and complicated RF circuits introduce more circuit noise and are more difficult to integrate into a small area.
Efforts have been made to reduce the repetitive use of RF hardware in a receiver connected to a smart antenna. One approach is to load reactive components to each antenna element to control the individual signal phase before combining. The drawback of this approach is that the signal phase and magnitude information is lost after combining, and advanced vector signal processing capability is not possible.
Another approach reduces the number of RF channels to one using a spatial multiplexing of local elements scheme. This scheme is disclosed in an article titled “A Smart Antenna Receiver Array Using A Single RF Channel And Digital Beamforming” by Fredrick et al., and is based on a signal element of the array being sequentially connected to signal processing circuitry in order to sample the incoming modulated carrier. The sampling rate is higher than the signal bandwidth so that the information of the original signal can be fully restored in post-processing stages using low pass filters.
The communications device 10 disclosed in the Fredrick et al. article is illustrated in FIG. 1, and includes an antenna array 12 comprising N elements 14, a PIN diode multiplexer 16 and a single RF channel. The single RF channel is defined between the PIN diode multiplexer 16 and an analog demultiplexer 18. The RF channel includes a low noise amplifier 20 and a mixer 22. A digital signal processor 24 is connected to the analog demultiplexer 18. The single RF channel advantageously reduces costs in terms of hardware and power consumption.
However, there are N channels of the signal which are sequentially multiplexed to form a single RF output. As FIG. 1 further illustrates, there is a separate circuit for each of the N channels between the analog demultiplexer 18 and the digital signal processor 24. Each separate circuit includes a low pass filter 26 and an analog-to-digital converter 28. These components have an impact on cost and power consumption of the receiver.